Conventional diesel fuel produced from crude oil (“petrodiesel”) contains a complex mixture of hydrocarbons which typically have 10-22 carbon atoms. These hydrocarbons include linear and branched alkanes, cycloalkanes and aromatic hydrocarbons. As a consequence of the crude oil source and the production process (particularly fractional distillation), petrodiesel typically contains up to 40 mass % of aromatic hydrocarbons, more usually 25-35 mass % of aromatic hydrocarbons. A significant proportion, usually in the order of 15-20 mass %, of the aromatic hydrocarbons are polycyclic (i.e. contain two or more aromatic rings). Such compounds are harmful to health (e.g. carcinogenic) and have poor combustion properties.
In most countries, diesel fuel must satisfy certain regulatory requirements before it can be sold. In the European Union, diesel fuels must comply with the EN 590 Standard. This requires diesel fuels to have various physical and chemical properties, including a density of 820-845 kg/m3 at 15° C. (as measured using test method EN ISO 3675 or EN ISO 12185), a maximum polycyclic aromatic hydrocarbon content of 11 mass %, and a cetane number of at least 51.0 (as measured using test method EN ISO 5165).
Renewable fuels derived from biological matter (“biofuels”) are gaining popularity as a more environmentally friendly alternative to conventional fossil fuels. Examples of biofuels include biodiesel, which is typically produced by transesterification of triglycerides contained in vegetable oils (e.g. soybean oil). This yields a mixture of fatty acid alkyl esters (e.g. fatty acid methyl ester (FAME)). Biodiesel can also be produced from animal fats (e.g. tallow).
FAME produced from a biological feedstock can be blended with petrodiesel, but in an amount of no more than 7 vol. % according to EN 590. This is due to the chemical and physical differences between FAME and petrodiesel. FAME contains ester groups, which are largely absent from petrodiesel. This difference is responsible for the inferior properties of FAME biodiesel at low temperatures and the inferior storage stability of FAME biodiesel. FAME biodiesel also has a tendency to degrade natural rubber components of automobiles (e.g. rubber gaskets).
FAME biodiesel has a significantly lower mass-based energy content than petrodiesel; the energy content of FAME biodiesel is typically about 38 MJ/kg, whereas the energy content of petrodiesel is typically about 43 MJ/kg. Taking into account the higher density of FAME biodiesel (approximately 885 kg/m3), the volume-based energy contents of FAME biodiesel and petrodiesel are typically about 34 MJ/1 and 36 MJ/l respectively.
A further disadvantage of FAME biodiesel is that its manufacture by transesterification of triglycerides produces a large quantity of glycerol. This is often an unwanted by-product due to low market demand. Moreover, purification of the crude glycerol is energy intensive.
A type of second generation biofuel is “biomass-to-liquid” (BTL) biofuel, which is produced from gasified biomass using the Fischer-Tropsch process. The gasified carbonaceous material reacts to produce a syngas (a mixture of carbon monoxide and hydrogen), which in turn undergoes polymerisation to produce hydrocarbons.
BTL biodiesel typically has a density of about 780 kg/m3, which is significantly lower than the density of petrodiesel. This means that the volume-based energy content of BTL biodiesel is only about 95% of that of petrodiesel.
An object of the present invention is to provide a renewable hydrocarbon composition which can be used as a fuel component.